The present application is an improvement over the foregoing applications in that it is directed to classes of molecules that provide switching from one state to a different state, characterized by a change in the optical properties, including color, of the molecules. In the case of color switching, the present invention turns ink or dye or pigment molecules into active opto-electronic devices that can be switched by an external electric field.
The present invention relates generally to optical devices whose functional length scales are measured in nanometers, and, more particularly, to classes of molecules that provide optical switching. Optical devices both of micrometer and nanometer scale may be constructed in accordance with the teachings herein.
The area of molecular electronics is in its infancy. To date, there have been two convincing demonstrations of molecules as electronic switches published in the technical literature; see, C. P. Collier et al., Science, Vol. 285, pp. 391-394 (Jul. 16, 1999) and C. P. Collier et al., Science, Vol. 289, pp. 1172-1175 (Aug. 18, 2000), but there is a great deal of speculation and interest within the scientific community surrounding this topic. In the published work, a molecule called a rotaxane or a catenane was trapped between two metal electrodes and caused to switch from an ON state to an OFF state by the application of a positive bias across the molecule. The ON and OFF states differed in resistivity by about a factor of 100 and 5, respectively, for the rotaxane and catenane.
The primary problem with the rotaxane was that it is an irreversible switch. It can only be toggled once. Thus, it can be used in a programmable read-only memory (PROM), but not in a RAM-like (random access memory) device nor in a reconfigurable system, such as a defect-tolerant communications and logic network. In addition, the rotaxane requires an oxidation and/or reduction reaction to occur before the switch can be toggled. This requires the expenditure of a significant amount of energy to toggle the switch. In addition, the large and complex nature of rotaxanes and related compounds potentially makes the switching times of the molecules slow. The primary problems with the catenanes are small ON-to-OFF ratio and a slow switching time.
Currently, there are a wide variety of known chromogenic materials that can provide optical switching in thin film form. These materials and their applications have been reviewed recently by C. B. Greenberg, Thin Solid Films, Vol. 251, pp. 81-93 (1994) and R. J. Mortimer, Chemical Society Reviews, Vol. 26, pp. 147-156 (1997). These materials are currently being studied for several applications, including active darkening of sunglasses, active darkening of windows for intelligent light and thermal management of buildings, and various types of optical displays, such as heads-up displays on the inside of windshields of automobiles or airplanes and eyeglass displays.
Despite their long history of great promise, there are very few photon gating devices made from the existing classes of electrochromic materials. This is because most of them require an oxidation-reduction reaction that involves the transport of ions, such as H+, Li+, or Na+ through some type of liquid or solid electrolyte. Finding the appropriate electrolyte is a major problem, as is the slow speed of any device that requires transport of ions. Furthermore, such reactions are extremely sensitive to background contamination, such as oxygen or other species, and thus degradation of the chromogenic electrodes is a major limitation.
In fact, for photonic switching applications such as a crossbar switch router for a fiber optic communications network, the lack of a suitable chromogenic material has forced companies to use very different approaches: (a) transform the optical signal into an electronic signal, perform the switching operation, and then transform back to an optical signal before launching into a fiber (this is the most frequent solution used today, but it is very inefficient and difficult for the electronics to keep up with the data rates of the optical system); (b) use a moving-mirror array made by micro-electromechanical (MEM) processing to switch optical data packets (this has the disadvantage that extremely high tolerances are required for the device, which makes it very expensive); and (c) using ink jet technology to push bubbles into a chamber to create a mirror to deflect an optical beam (this approach again requires precision manufacturing and the switching time is slow).
Thus, what is needed is a molecular system that avoids chemical oxidation and/or reduction, permits reasonably rapid switching from a first state to a second, is reversible to permit real-time or video rate display applications, and can be used in a variety of optical devices.
In accordance with the present invention, a molecular system is provided for optical switching. The molecular system has an electric field induced band gap change that occurs via one of the following mechanisms:
(1) molecular conformation change or an isomerization;
(2) change of extended conjugation via chemical bonding change to change the band gap; or
(3) molecular folding or stretching.
Changing of extended conjugation via chemical bonding change to change the band gap may be accomplished in one of the following ways:
(a) charge separation or recombination accompanied by increasing or decreasing band localization; or
(b) change of extended conjugation via charge separation or recombination and xcfx80-bond breaking or formation.
The present invention provides, e.g., optical switches that can be assembled easily to make displays, electronic books, rewrittable media, electronic lenses, electrically-controlled tinting for windows and mirrors, optical crossbar switches for fiber optic communications, and more. Such applications are discussed elsewhere, and are not germane to the present invention, except to the extent that the optical switch of the present invention is employed in the construction of apparatus of such applications.
The present invention introduces several new types of switching mechanism: (1) an electric (E) field induced rotation of at least one rotatable section (rotor) of a molecule to change the band gap of the molecule; (2) E-field induced charge separation or re-combination of the molecule via chemical bonding change to change the band gap; (3) E-field induced band gap change via molecule folding or stretching. These devices are generically considered to be electric field devices, and are to be distinguished from earlier embodiments (described in the above-mentioned related patent applications and patent) that are directed to electrochemical devices.
The present invention also introduces the capability of using molecules for optical switches, in which the molecules change color when changing state. This property can be used for a wide variety of display devices or any other application enabled by a material that can change color or transform from transparent to colored.
Thus, the molecule is not oxidized nor reduced in the toggling of the switch. Also, the part of the molecule that moves is quite small, so the switching time should be very fast. Also, the molecules are much simpler and thus easier and cheaper to make than the rotaxanes, catenanes, and related compounds.